Outputs sometimes dropped from Output widget when updated in subthreads

[rawlins]

Description

I'm experiencing weird interactions with threading and Output widgets in Jupyter, that only happen on the second or later output widget created in a notebook. Essentially, some (but not all) output in a subthread that should be in the widget, ends up orphaned from the widget, leading to out of order output sequences and output that is missing altogether (when clear_display has been used, see note 2 below).

I believe this is related to changes in ipython/ipykernel#1451 but I've somewhat reached my diagnostic limits on this for the moment. Anecdotally, reverting to ipykernel 6 is reported to fix the downstream bugs I'm chasing.

Reproduce

Consider the following minimal example when run in a Jupyter notebook cell:

import threading
import ipywidgets as widgets
import time
current_cell_parent = get_ipython().get_parent()

out = widgets.Output(layout={'border': '1px solid black'})
display(out)
with out:
    print("initial")

def thread_task():
    get_ipython().set_parent(current_cell_parent)
    for i in range(5):
        with out:
            print(f"...{i}")
            time.sleep(0.5)

t = threading.Thread(target=thread_task)
t.start()
t.join()

with out:
    print(f"final text")

The expected final output of this cell, and what happens the first time you run it, would be the following, within a 1px border:

initial
...0
...1
...2
...3
...4
final text

At this point out.outputs is as expected:

({'name': 'stdout',
  'text': 'initial\n...0\n...1\n...2\n...3\n...4\nfinal text\n',
  'output_type': 'stream'},)

To replicate the bug, just rerun the cell with the above code at least a second time. What I am seeing after the code finishes is the following, with the border only around the first three lines:

initial
...0
final text
...1
...2
...3
...4

At this point, out.outputs is:

({'name': 'stdout',
  'text': 'initial\n...0\nfinal text\n',
  'output_type': 'stream'},)

That is, it appears on reruns that the outputs 1-4 have been "orphaned" from the widget / do not render within the widget.

Note 1: I found this bug while trying to track down Textualize/rich#3868 and pymc-devs/pymc#7757. The minimal example above is essentially those bugs abstracted from their package context with the addition of the set_parent call (per ipython/ipykernel#1451). Without the set_parent call in the thread, a different but closely related behavior happens (this is the downstream bug). On the first run, everything works as expected. On the second and later runs, only the ...0 output appears from the thread, and outputs 1-4 are dropped altogether. So I suspect the root cause is the same; I'm not sure what the expected behavior is without the set_parent call in ipykernel 7+ though; my impression from the linked ipykernel PR is that this is obligatory (maybe I'm wrong).

Note 2: if clear_output is called in the thread's display loop and the final output (simulating a typical pattern for things like rich's progress bars), the final output doesn't render at all, though it is present in out.outputs. I haven't focused on this case because I'm guessing it's a side effect of the real bug. I can provide a minimal case for this though if that would be helpful.

Note 3: Restarting the kernel will restore the expected behavior (once). I haven't found any package reloading scheme that fixes it.

Expected behavior

Stable behavior when running the cell multiple times. (More generally, this happens any time there are multiple output widgets, so stable behavior between widgets.)

Context

• ipywidgets version 8.1.8
• Operating System and version: MacOS 15.2
• Browser and version: Firefox 150.0

Troubleshoot Output

$PATH:
	/opt/local/libexec/ccache
	/Users/advil/miniforge3/envs/ipywidgets-test/bin
	/Users/advil/miniforge3/condabin
	/opt/local/libexec/gnubin
	/opt/local/bin
	/opt/local/sbin
	/usr/local/bin
	/System/Cryptexes/App/usr/bin
	/usr/bin
	/bin
	/usr/sbin
	/sbin
	/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin
	/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin
	/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin
	/Library/Apple/usr/bin
	/Library/TeX/texbin
sys.path:

/Users/advil/miniforge3/envs/ipywidgets-test/bin

/Users/advil/miniforge3/envs/ipywidgets-test/lib/python314.zip

/Users/advil/miniforge3/envs/ipywidgets-test/lib/python3.14

/Users/advil/miniforge3/envs/ipywidgets-test/lib/python3.14/lib-dynload

/Users/advil/miniforge3/envs/ipywidgets-test/lib/python3.14/site-packages
sys.executable:

/Users/advil/miniforge3/envs/ipywidgets-test/bin/python3.14
sys.version:

3.14.5 | packaged by conda-forge | (main, May 20 2026, 00:45:47) [Clang 20.1.8 ]
platform.platform():

macOS-15.2-arm64-arm-64bit-Mach-O
which -a jupyter:

/Users/advil/miniforge3/envs/ipywidgets-test/bin/jupyter
pip list:

Package                   Version

------------------------- -----------

anyio                     4.13.0

appnope                   0.1.4

argon2-cffi               25.1.0

argon2-cffi-bindings      25.1.0

arrow                     1.4.0

asttokens                 3.0.1

async-lru                 2.3.0

attrs                     26.1.0

babel                     2.18.0

beautifulsoup4            4.14.3

bleach                    6.3.0

Brotli                    1.2.0

cached-property           1.5.2

certifi                   2026.5.20

cffi                      2.0.0

charset-normalizer        3.4.7

comm                      0.2.3

debugpy                   1.8.20

decorator                 5.3.1

defusedxml                0.7.1

exceptiongroup            1.3.1

executing                 2.2.1

fastjsonschema            2.21.2

fqdn                      1.5.1

h11                       0.16.0

h2                        4.3.0

hpack                     4.1.0

httpcore                  1.0.9

httpx                     0.28.1

hyperframe                6.1.0

idna                      3.17

importlib_metadata        9.0.0

ipykernel                 7.2.0

ipython                   9.14.0

ipython_pygments_lexers   1.1.1

ipywidgets                8.1.8

isoduration               20.11.0

jedi                      0.19.2

Jinja2                    3.1.6

json5                     0.14.0

jsonpointer               3.1.1

jsonschema                4.26.0

jsonschema-specifications 2025.9.1

jupyter_client            8.8.0

jupyter_core              5.9.1

jupyter-events            0.12.1

jupyter-lsp               2.3.1

jupyter_server            2.19.0

jupyter_server_terminals  0.5.4

jupyterlab                4.5.7

jupyterlab_pygments       0.3.0

jupyterlab_server         2.28.0

jupyterlab_widgets        3.0.16

lark                      1.3.1

MarkupSafe                3.0.3

matplotlib-inline         0.2.2

mistune                   3.2.1

nbclient                  0.10.4

nbconvert                 7.17.1

nbformat                  5.10.4

nest_asyncio              1.6.0

notebook_shim             0.2.4

overrides                 7.7.0

packaging                 26.2

pandocfilters             1.5.0

parso                     0.8.7

pexpect                   4.9.0

pip                       26.1.1

platformdirs              4.10.0

prometheus_client         0.25.0

prompt_toolkit            3.0.52

psutil                    7.2.2

ptyprocess                0.7.0

pure_eval                 0.2.3

pycparser                 3.0

Pygments                  2.20.0

pyobjc-core               12.1

pyobjc-framework-Cocoa    12.1

PySocks                   1.7.1

python-dateutil           2.9.0.post0

python-json-logger        3.2.1

PyYAML                    6.0.3

pyzmq                     27.1.0

referencing               0.37.0

requests                  2.34.2

rfc3339_validator         0.1.4

rfc3986-validator         0.1.1

rfc3987-syntax            1.1.0

rpds-py                   2026.5.1

Send2Trash                2.1.0

setuptools                82.0.1

six                       1.17.0

sniffio                   1.3.1

soupsieve                 2.8.4

stack_data                0.6.3

terminado                 0.18.1

tinycss2                  1.4.0

tomli                     2.4.1

tornado                   6.5.6

traitlets                 5.15.0

typing_extensions         4.15.0

typing_utils              0.1.0

tzdata                    2026.2

uri-template              1.3.0

urllib3                   2.7.0

wcwidth                   0.7.0

webcolors                 25.10.0

webencodings              0.5.1

websocket-client          1.9.0

widgetsnbextension        4.0.15

zipp                      4.1.0
conda list:

# packages in environment at /Users/advil/miniforge3/envs/ipywidgets-test:

#

# Name                    Version                   Build  Channel

_python_abi3_support      1.0                  hd8ed1ab_2    conda-forge

anyio                     4.13.0             pyhcf101f3_0    conda-forge

appnope                   0.1.4              pyhd8ed1ab_1    conda-forge

argon2-cffi               25.1.0             pyhd8ed1ab_0    conda-forge

argon2-cffi-bindings      25.1.0          py314h0612a62_2    conda-forge

arrow                     1.4.0              pyhcf101f3_0    conda-forge

asttokens                 3.0.1              pyhd8ed1ab_0    conda-forge

async-lru                 2.3.0              pyhcf101f3_0    conda-forge

attrs                     26.1.0             pyhcf101f3_0    conda-forge

babel                     2.18.0             pyhcf101f3_1    conda-forge

backports.zstd            1.5.0           py314h680f03e_0    conda-forge

beautifulsoup4            4.14.3             pyha770c72_0    conda-forge

bleach                    6.3.0              pyhcf101f3_1    conda-forge

bleach-with-css           6.3.0                hbca2aae_1    conda-forge

brotli-python             1.2.0           py314h3daef5d_1    conda-forge

bzip2                     1.0.8                hd037594_9    conda-forge

ca-certificates           2026.5.20            hbd8a1cb_0    conda-forge

cached-property           1.5.2                hd8ed1ab_1    conda-forge

cached_property           1.5.2              pyha770c72_1    conda-forge

certifi                   2026.5.20          pyhd8ed1ab_0    conda-forge

cffi                      2.0.0           py314h44086f9_1    conda-forge

charset-normalizer        3.4.7              pyhd8ed1ab_0    conda-forge

comm                      0.2.3              pyhe01879c_0    conda-forge

cpython                   3.14.5          py314hd8ed1ab_100    conda-forge

debugpy                   1.8.20          py314he609de1_0    conda-forge

decorator                 5.3.1              pyhd8ed1ab_0    conda-forge

defusedxml                0.7.1              pyhd8ed1ab_0    conda-forge

exceptiongroup            1.3.1              pyhd8ed1ab_0    conda-forge

executing                 2.2.1              pyhd8ed1ab_0    conda-forge

fqdn                      1.5.1              pyhd8ed1ab_1    conda-forge

h11                       0.16.0             pyhcf101f3_1    conda-forge

h2                        4.3.0              pyhcf101f3_0    conda-forge

hpack                     4.1.0              pyhd8ed1ab_0    conda-forge

httpcore                  1.0.9              pyh29332c3_0    conda-forge

httpx                     0.28.1             pyhd8ed1ab_0    conda-forge

hyperframe                6.1.0              pyhd8ed1ab_0    conda-forge

idna                      3.17               pyhcf101f3_0    conda-forge

importlib-metadata        9.0.0              pyhcf101f3_0    conda-forge

ipykernel                 7.2.0              pyh5552912_1    conda-forge

ipython                   9.14.0             pyh53cf698_0    conda-forge

ipython_pygments_lexers   1.1.1              pyhd8ed1ab_0    conda-forge

ipywidgets                8.1.8                    pypi_0    pypi

isoduration               20.11.0            pyhd8ed1ab_1    conda-forge

jedi                      0.19.2             pyhd8ed1ab_1    conda-forge

jinja2                    3.1.6              pyhcf101f3_1    conda-forge

json5                     0.14.0             pyhd8ed1ab_0    conda-forge

jsonpointer               3.1.1              pyhcf101f3_0    conda-forge

jsonschema                4.26.0             pyhcf101f3_0    conda-forge

jsonschema-specifications 2025.9.1           pyhcf101f3_0    conda-forge

jsonschema-with-format-nongpl 4.26.0               hcf101f3_0    conda-forge

jupyter-lsp               2.3.1              pyhcf101f3_0    conda-forge

jupyter_client            8.8.0              pyhcf101f3_0    conda-forge

jupyter_core              5.9.1              pyhc90fa1f_0    conda-forge

jupyter_events            0.12.1             pyhcf101f3_0    conda-forge

jupyter_server            2.19.0             pyhcf101f3_0    conda-forge

jupyter_server_terminals  0.5.4              pyhcf101f3_0    conda-forge

jupyterlab                4.5.7              pyhd8ed1ab_0    conda-forge

jupyterlab-widgets        3.0.16                   pypi_0    pypi

jupyterlab_pygments       0.3.0              pyhd8ed1ab_2    conda-forge

jupyterlab_server         2.28.0             pyhcf101f3_0    conda-forge

krb5                      1.22.2               h385eeb1_0    conda-forge

lark                      1.3.1              pyhd8ed1ab_0    conda-forge

libcxx                    22.1.6               h55c6f16_0    conda-forge

libedit                   3.1.20250104    pl5321hafb1f1b_0    conda-forge

libexpat                  2.8.1                hf6b4638_0    conda-forge

libffi                    3.5.2                hcf2aa1b_0    conda-forge

liblzma                   5.8.3                h8088a28_0    conda-forge

libmpdec                  4.0.0                h84a0fba_1    conda-forge

libsodium                 1.0.22               h1a92334_1    conda-forge

libsqlite                 3.53.1               h1b79a29_0    conda-forge

libzlib                   1.3.2                h8088a28_2    conda-forge

markupsafe                3.0.3           py314h6e9b3f0_1    conda-forge

matplotlib-inline         0.2.2              pyhd8ed1ab_0    conda-forge

mistune                   3.2.1              pyhcf101f3_0    conda-forge

nbclient                  0.10.4             pyhd8ed1ab_0    conda-forge

nbconvert-core            7.17.1             pyhcf101f3_0    conda-forge

nbformat                  5.10.4             pyhd8ed1ab_1    conda-forge

ncurses                   6.6                  h1d4f5a5_0    conda-forge

nest-asyncio              1.6.0              pyhd8ed1ab_1    conda-forge

notebook-shim             0.2.4              pyhd8ed1ab_1    conda-forge

openssl                   3.6.2                hd24854e_0    conda-forge

overrides                 7.7.0              pyhd8ed1ab_1    conda-forge

packaging                 26.2               pyhc364b38_0    conda-forge

pandocfilters             1.5.0              pyhd8ed1ab_0    conda-forge

parso                     0.8.7              pyhcf101f3_0    conda-forge

pexpect                   4.9.0              pyhd8ed1ab_1    conda-forge

pip                       26.1.1             pyh145f28c_0    conda-forge

platformdirs              4.10.0             pyhcf101f3_0    conda-forge

prometheus_client         0.25.0             pyhd8ed1ab_0    conda-forge

prompt-toolkit            3.0.52             pyha770c72_0    conda-forge

psutil                    7.2.2           py314ha14b1ff_0    conda-forge

ptyprocess                0.7.0              pyhd8ed1ab_1    conda-forge

pure_eval                 0.2.3              pyhd8ed1ab_1    conda-forge

pycparser                 3.0                pyhcf101f3_0    conda-forge

pygments                  2.20.0             pyhd8ed1ab_0    conda-forge

pyobjc-core               12.1            py314h3a4d195_0    conda-forge

pyobjc-framework-cocoa    12.1            py314h36abed7_0    conda-forge

pysocks                   1.7.1              pyha55dd90_7    conda-forge

python                    3.14.5          h4c637c5_100_cp314    conda-forge

python-dateutil           2.9.0.post0        pyhe01879c_2    conda-forge

python-fastjsonschema     2.21.2             pyhe01879c_0    conda-forge

python-gil                3.14.5             h4df99d1_100    conda-forge

python-json-logger        3.2.1              pyh332efcf_0    conda-forge

python-tzdata             2026.2             pyhd8ed1ab_0    conda-forge

python_abi                3.14                    8_cp314    conda-forge

pyyaml                    6.0.3           py314h6e9b3f0_1    conda-forge

pyzmq                     27.1.0          py312h022ad19_3    conda-forge

readline                  8.3                  h46df422_0    conda-forge

referencing               0.37.0             pyhcf101f3_0    conda-forge

requests                  2.34.2             pyhcf101f3_0    conda-forge

rfc3339-validator         0.1.4              pyhd8ed1ab_1    conda-forge

rfc3986-validator         0.1.1              pyh9f0ad1d_0    conda-forge

rfc3987-syntax            1.1.0              pyhe01879c_1    conda-forge

rpds-py                   2026.5.1        py314he1d1ac0_0    conda-forge

send2trash                2.1.0              pyh5552912_1    conda-forge

setuptools                82.0.1             pyh332efcf_0    conda-forge

six                       1.17.0             pyhe01879c_1    conda-forge

sniffio                   1.3.1              pyhd8ed1ab_2    conda-forge

soupsieve                 2.8.4              pyhd8ed1ab_0    conda-forge

stack_data                0.6.3              pyhd8ed1ab_1    conda-forge

terminado                 0.18.1             pyhc90fa1f_1    conda-forge

tinycss2                  1.4.0              pyhd8ed1ab_0    conda-forge

tk                        8.6.13               h010d191_3    conda-forge

tomli                     2.4.1              pyhcf101f3_0    conda-forge

tornado                   6.5.6           py314h6c2aa35_0    conda-forge

traitlets                 5.15.0             pyhcf101f3_0    conda-forge

typing-extensions         4.15.0               h396c80c_0    conda-forge

typing_extensions         4.15.0             pyhcf101f3_0    conda-forge

typing_utils              0.1.0              pyhd8ed1ab_1    conda-forge

tzdata                    2025c                hc9c84f9_1    conda-forge

uri-template              1.3.0              pyhd8ed1ab_1    conda-forge

urllib3                   2.7.0              pyhd8ed1ab_0    conda-forge

wcwidth                   0.7.0              pyhd8ed1ab_0    conda-forge

webcolors                 25.10.0            pyhd8ed1ab_0    conda-forge

webencodings              0.5.1              pyhd8ed1ab_3    conda-forge

websocket-client          1.9.0              pyhd8ed1ab_0    conda-forge

widgetsnbextension        4.0.15                   pypi_0    pypi

yaml                      0.2.5                h925e9cb_3    conda-forge

zeromq                    4.3.5               h10816f8_11    conda-forge

zipp                      4.1.0              pyhcf101f3_0    conda-forge

zstd                      1.5.7                hbf9d68e_6    conda-forge
conda env:

name: ipywidgets-test

channels:

- conda-forge

dependencies:

- _python_abi3_support=1.0=hd8ed1ab_2

- anyio=4.13.0=pyhcf101f3_0

- appnope=0.1.4=pyhd8ed1ab_1

- argon2-cffi=25.1.0=pyhd8ed1ab_0

- argon2-cffi-bindings=25.1.0=py314h0612a62_2

- arrow=1.4.0=pyhcf101f3_0

- asttokens=3.0.1=pyhd8ed1ab_0

- async-lru=2.3.0=pyhcf101f3_0

- attrs=26.1.0=pyhcf101f3_0

- babel=2.18.0=pyhcf101f3_1

- backports.zstd=1.5.0=py314h680f03e_0

- beautifulsoup4=4.14.3=pyha770c72_0

- bleach=6.3.0=pyhcf101f3_1

- bleach-with-css=6.3.0=hbca2aae_1

- brotli-python=1.2.0=py314h3daef5d_1

- bzip2=1.0.8=hd037594_9

- ca-certificates=2026.5.20=hbd8a1cb_0

- cached-property=1.5.2=hd8ed1ab_1

- cached_property=1.5.2=pyha770c72_1

- certifi=2026.5.20=pyhd8ed1ab_0

- cffi=2.0.0=py314h44086f9_1

- charset-normalizer=3.4.7=pyhd8ed1ab_0

- comm=0.2.3=pyhe01879c_0

- cpython=3.14.5=py314hd8ed1ab_100

- debugpy=1.8.20=py314he609de1_0

- decorator=5.3.1=pyhd8ed1ab_0

- defusedxml=0.7.1=pyhd8ed1ab_0

- exceptiongroup=1.3.1=pyhd8ed1ab_0

- executing=2.2.1=pyhd8ed1ab_0

- fqdn=1.5.1=pyhd8ed1ab_1

- h11=0.16.0=pyhcf101f3_1

- h2=4.3.0=pyhcf101f3_0

- hpack=4.1.0=pyhd8ed1ab_0

- httpcore=1.0.9=pyh29332c3_0

- httpx=0.28.1=pyhd8ed1ab_0

- hyperframe=6.1.0=pyhd8ed1ab_0

- idna=3.17=pyhcf101f3_0

- importlib-metadata=9.0.0=pyhcf101f3_0

- ipykernel=7.2.0=pyh5552912_1

- ipython=9.14.0=pyh53cf698_0

- ipython_pygments_lexers=1.1.1=pyhd8ed1ab_0

- isoduration=20.11.0=pyhd8ed1ab_1

- jedi=0.19.2=pyhd8ed1ab_1

- jinja2=3.1.6=pyhcf101f3_1

- json5=0.14.0=pyhd8ed1ab_0

- jsonpointer=3.1.1=pyhcf101f3_0

- jsonschema=4.26.0=pyhcf101f3_0

- jsonschema-specifications=2025.9.1=pyhcf101f3_0

- jsonschema-with-format-nongpl=4.26.0=hcf101f3_0

- jupyter-lsp=2.3.1=pyhcf101f3_0

- jupyter_client=8.8.0=pyhcf101f3_0

- jupyter_core=5.9.1=pyhc90fa1f_0

- jupyter_events=0.12.1=pyhcf101f3_0

- jupyter_server=2.19.0=pyhcf101f3_0

- jupyter_server_terminals=0.5.4=pyhcf101f3_0

- jupyterlab=4.5.7=pyhd8ed1ab_0

- jupyterlab_pygments=0.3.0=pyhd8ed1ab_2

- jupyterlab_server=2.28.0=pyhcf101f3_0

- krb5=1.22.2=h385eeb1_0

- lark=1.3.1=pyhd8ed1ab_0

- libcxx=22.1.6=h55c6f16_0

- libedit=3.1.20250104=pl5321hafb1f1b_0

- libexpat=2.8.1=hf6b4638_0

- libffi=3.5.2=hcf2aa1b_0

- liblzma=5.8.3=h8088a28_0

- libmpdec=4.0.0=h84a0fba_1

- libsodium=1.0.22=h1a92334_1

- libsqlite=3.53.1=h1b79a29_0

- libzlib=1.3.2=h8088a28_2

- markupsafe=3.0.3=py314h6e9b3f0_1

- matplotlib-inline=0.2.2=pyhd8ed1ab_0

- mistune=3.2.1=pyhcf101f3_0

- nbclient=0.10.4=pyhd8ed1ab_0

- nbconvert-core=7.17.1=pyhcf101f3_0

- nbformat=5.10.4=pyhd8ed1ab_1

- ncurses=6.6=h1d4f5a5_0

- nest-asyncio=1.6.0=pyhd8ed1ab_1

- notebook-shim=0.2.4=pyhd8ed1ab_1

- openssl=3.6.2=hd24854e_0

- overrides=7.7.0=pyhd8ed1ab_1

- packaging=26.2=pyhc364b38_0

- pandocfilters=1.5.0=pyhd8ed1ab_0

- parso=0.8.7=pyhcf101f3_0

- pexpect=4.9.0=pyhd8ed1ab_1

- pip=26.1.1=pyh145f28c_0

- platformdirs=4.10.0=pyhcf101f3_0

- prometheus_client=0.25.0=pyhd8ed1ab_0

- prompt-toolkit=3.0.52=pyha770c72_0

- psutil=7.2.2=py314ha14b1ff_0

- ptyprocess=0.7.0=pyhd8ed1ab_1

- pure_eval=0.2.3=pyhd8ed1ab_1

- pycparser=3.0=pyhcf101f3_0

- pygments=2.20.0=pyhd8ed1ab_0

- pyobjc-core=12.1=py314h3a4d195_0

- pyobjc-framework-cocoa=12.1=py314h36abed7_0

- pysocks=1.7.1=pyha55dd90_7

- python=3.14.5=h4c637c5_100_cp314

- python-dateutil=2.9.0.post0=pyhe01879c_2

- python-fastjsonschema=2.21.2=pyhe01879c_0

- python-gil=3.14.5=h4df99d1_100

- python-json-logger=3.2.1=pyh332efcf_0

- python-tzdata=2026.2=pyhd8ed1ab_0

- python_abi=3.14=8_cp314

- pyyaml=6.0.3=py314h6e9b3f0_1

- pyzmq=27.1.0=py312h022ad19_3

- readline=8.3=h46df422_0

- referencing=0.37.0=pyhcf101f3_0

- requests=2.34.2=pyhcf101f3_0

- rfc3339-validator=0.1.4=pyhd8ed1ab_1

- rfc3986-validator=0.1.1=pyh9f0ad1d_0

- rfc3987-syntax=1.1.0=pyhe01879c_1

- rpds-py=2026.5.1=py314he1d1ac0_0

- send2trash=2.1.0=pyh5552912_1

- setuptools=82.0.1=pyh332efcf_0

- six=1.17.0=pyhe01879c_1

- sniffio=1.3.1=pyhd8ed1ab_2

- soupsieve=2.8.4=pyhd8ed1ab_0

- stack_data=0.6.3=pyhd8ed1ab_1

- terminado=0.18.1=pyhc90fa1f_1

- tinycss2=1.4.0=pyhd8ed1ab_0

- tk=8.6.13=h010d191_3

- tomli=2.4.1=pyhcf101f3_0

- tornado=6.5.6=py314h6c2aa35_0

- traitlets=5.15.0=pyhcf101f3_0

- typing-extensions=4.15.0=h396c80c_0

- typing_extensions=4.15.0=pyhcf101f3_0

- typing_utils=0.1.0=pyhd8ed1ab_1

- tzdata=2025c=hc9c84f9_1

- uri-template=1.3.0=pyhd8ed1ab_1

- urllib3=2.7.0=pyhd8ed1ab_0

- wcwidth=0.7.0=pyhd8ed1ab_0

- webcolors=25.10.0=pyhd8ed1ab_0

- webencodings=0.5.1=pyhd8ed1ab_3

- websocket-client=1.9.0=pyhd8ed1ab_0

- yaml=0.2.5=h925e9cb_3

- zeromq=4.3.5=h10816f8_11

- zipp=4.1.0=pyhcf101f3_0

- zstd=1.5.7=hbf9d68e_6

- pip:

- ipywidgets==8.1.8

- jupyterlab-widgets==3.0.16

- widgetsnbextension==4.0.15

prefix: /Users/advil/miniforge3/envs/ipywidgets-test

Command Line Output

[I 2026-05-30 14:57:24.998 ServerApp] jupyter_lsp | extension was successfully linked.
[I 2026-05-30 14:57:25.000 ServerApp] jupyter_server_terminals | extension was successfully linked.
[I 2026-05-30 14:57:25.001 ServerApp] jupyterlab | extension was successfully linked.
[I 2026-05-30 14:57:25.257 ServerApp] notebook_shim | extension was successfully linked.
[I 2026-05-30 14:57:25.274 ServerApp] notebook_shim | extension was successfully loaded.
[I 2026-05-30 14:57:25.275 ServerApp] jupyter_lsp | extension was successfully loaded.
[I 2026-05-30 14:57:25.276 ServerApp] jupyter_server_terminals | extension was successfully loaded.
[I 2026-05-30 14:57:25.277 LabApp] JupyterLab extension loaded from /Users/advil/miniforge3/envs/ipywidgets-test/lib/python3.14/site-packages/jupyterlab
[I 2026-05-30 14:57:25.277 LabApp] JupyterLab application directory is /Users/advil/miniforge3/envs/ipywidgets-test/share/jupyter/lab
[I 2026-05-30 14:57:25.278 LabApp] Extension Manager is 'pypi'.
[I 2026-05-30 14:57:25.301 ServerApp] jupyterlab | extension was successfully loaded.
[I 2026-05-30 14:57:25.302 ServerApp] The port 8888 is already in use, trying another port.
[I 2026-05-30 14:57:25.302 ServerApp] The port 8889 is already in use, trying another port.
[I 2026-05-30 14:57:25.302 ServerApp] Serving notebooks from local directory: /Users/advil/Dropbox/Projects/lexicon/proto-26/notebooks/tmp
[I 2026-05-30 14:57:25.302 ServerApp] Jupyter Server 2.19.0 is running at:
[I 2026-05-30 14:57:25.302 ServerApp] http://localhost:8890/lab?token=5e9aea8c851e33769d4aa944f7d35931d29d2d50469144aa
[I 2026-05-30 14:57:25.302 ServerApp]     http://127.0.0.1:8890/lab?token=5e9aea8c851e33769d4aa944f7d35931d29d2d50469144aa
[I 2026-05-30 14:57:25.302 ServerApp] Use Control-C to stop this server and shut down all kernels (twice to skip confirmation).
[C 2026-05-30 14:57:25.305 ServerApp] 
To access the server, open this file in a browser:
    file:/Users/advil/Library/Jupyter/runtime/jpserver-7342-open.html
Or copy and paste one of these URLs:
    http://localhost:8890/lab?token=5e9aea8c851e33769d4aa944f7d35931d29d2d50469144aa
    http://127.0.0.1:8890/lab?token=5e9aea8c851e33769d4aa944f7d35931d29d2d50469144aa

[I 2026-05-30 14:57:25.316 ServerApp] Skipped non-installed server(s): basedpyright, bash-language-server, dockerfile-language-server-nodejs, javascript-typescript-langserver, jedi-language-server, julia-language-server, pyrefly, pyright, python-language-server, python-lsp-server, r-languageserver, sql-language-server, texlab, typescript-language-server, unified-language-server, vscode-css-languageserver-bin, vscode-html-languageserver-bin, vscode-json-languageserver-bin, yaml-language-server

[W 2026-05-30 14:57:27.718 LabApp] Could not determine jupyterlab build status without nodejs

[I 2026-05-30 14:57:32.055 ServerApp] Kernel started: 83909f97-efe5-446c-b2a0-723a29a8aad6

[I 2026-05-30 14:57:32.693 ServerApp] Adapting from protocol version 5.3 (kernel 83909f97-efe5-446c-b2a0-723a29a8aad6) to 5.4 (client).

[I 2026-05-30 14:57:32.693 ServerApp] Connecting to kernel 83909f97-efe5-446c-b2a0-723a29a8aad6.

[W 2026-05-30 14:57:32.695 ServerApp] The websocket_ping_timeout (90000) cannot be longer than the websocket_ping_interval (30000).

Setting websocket_ping_timeout=30000

[I 2026-05-30 14:57:32.697 ServerApp] Adapting from protocol version 5.3 (kernel 83909f97-efe5-446c-b2a0-723a29a8aad6) to 5.4 (client).

[I 2026-05-30 14:57:32.697 ServerApp] Connecting to kernel 83909f97-efe5-446c-b2a0-723a29a8aad6.

[I 2026-05-30 14:57:32.700 ServerApp] Adapting from protocol version 5.3 (kernel 83909f97-efe5-446c-b2a0-723a29a8aad6) to 5.4 (client).

[I 2026-05-30 14:57:32.701 ServerApp] Connecting to kernel 83909f97-efe5-446c-b2a0-723a29a8aad6.

[W 2026-05-30 14:57:32.710 ServerApp] Got events for closed stream <zmq.eventloop.zmqstream.ZMQStream object at 0x10b199090>

[I 2026-05-30 14:59:31.974 ServerApp] Saving file at /Untitled.ipynb

Nothing in the browser console log.

If using JupyterLab

• JupyterLab version: 4.5.7

Installed Labextensions

        jupyterlab_pygments v0.3.0 enabled OK (python, jupyterlab_pygments)
        @jupyter-widgets/jupyterlab-manager v5.0.15 enabled OK (python, jupyterlab_widgets)

[rawlins]

I dug into this a bit more, and I have a possible fix. I say "possible" because verifying this would require deeper knowledge of ipykernel and how it interacts with Output.__enter__ than I currently have; hence me putting the fix here rather than a PR.

Essentially: I think that the now-correct way to get the parent msg_id is via get_ipython().get_parent(), rather than get_ipython().kernel.get_parent() as Output now uses. I have no idea why the latter shows the behavior pattern it does, but it consistently returns a comm_msg message parent on later outputs from a thread after the first run, rather than the execute_request parent that would lead to correct widget behavior. Getting the parent via the shell object always returns the correct parent in my testing. Unfortunately neither the documentation around the two versions of get_parent nor what there is around ipython/ipykernel#1451 ever really explain how/when to use these functions. If someone knowledgeable advises me that this solution looks correct, I would be happy to turn it into a PR.

Anyways, here is a proof of concept that fixes the issue bug; under all circumstances threaded output to a widget will appear in the widget with this subclass:

import ipywidgets as widgets
import time

class TestOutput(widgets.Output):
    __counter = 0

    def __enter__(self):
        """Called upon entering output widget context manager."""
        ip = get_ipython()
        self._flush()
        anchor = None
        if ip and getattr(ip, "get_parent", None) is not None:
            # *change*
            # as of ipykernel 7, get_parent on the shell, not the kernel, appears to be
            # the correct way to find the right msg_id. This code is not tested on
            # ipykernel <7...
            anchor = ip
        elif ip and getattr(ip, "kernel", None) is not None:
            # possibly unreachable, was used pre ipykernel 6
            anchor = ip.kernel
        elif self.comm is not None and getattr(self.comm, 'kernel', None) is not None:
            # (very) old shell or no shell at all
            anchor = self.comm.kernel
    
        if anchor:
            parent = None
            if hasattr(anchor, "get_parent"):
                parent = anchor.get_parent()
            elif hasattr(anchor, "_parent_header"):
                # ipykernel < 6: kernel._parent_header is the parent *request*
                parent = anchor._parent_header

            if parent and parent.get("header"):
                self.msg_id = parent["header"]["msg_id"]
                self.__counter += 1

    def __exit__(self, etype, evalue, tb):
        # need to override this b/c of name mangling on __counter, no changes
        """Called upon exiting output widget context manager."""
        kernel = None
        if etype is not None:
            ip = get_ipython()
            if ip:
                kernel = ip
                ip.showtraceback((etype, evalue, tb), tb_offset=0)
            elif (self.comm is not None and
                    getattr(self.comm, "kernel", None) is not None and
                    # Check if it's ipykernel
                    getattr(self.comm.kernel, "send_response", None) is not None):
                kernel = self.comm.kernel
                kernel.send_response(kernel.iopub_socket,
                                     u'error',
                                     {
                    u'traceback': ["".join(traceback.format_exception(etype, evalue, tb))],
                    u'evalue': repr(evalue.args),
                    u'ename': etype.__name__
                    })
        self._flush()
        self.__counter -= 1
        if self.__counter == 0:
            self.msg_id = ''
        # suppress exceptions when in IPython, since they are shown above,
        # otherwise let someone else handle it
        return True if kernel else None

Extra info: here are examples of the parent found via the two different means. On the first run, unconditionally, get_ipython().kernel.get_parent() from a thread finds a parent with a header like this; main thread calls as well as the first use of with out: in a thread also find this parent, and (this is the fix) get_ipython().get_parent() always finds it:

{'date': datetime.datetime(2026, 5, 31, 19, 10, 6, 143000, tzinfo=tzutc()),
 'msg_id': '996f68e7-f732-4853-a049-c7ac980958a8',
 'msg_type': 'execute_request',
 'session': '25ff9cbd-12d5-42bb-ad42-85d0a8709dac',
 'username': '',
 'subshell_id': None,
 'version': '5.2'}

In a thread, on the second run after with out: has been used once, this is the parent's header. Using the msg_id here displays output in the cell but not the widget.

{'date': datetime.datetime(2026, 5, 31, 19, 9, 27, 986000, tzinfo=tzutc()),
 'msg_id': '84479363-2a1e-4be1-af1c-f5564673c82c',
 'msg_type': 'comm_msg',
 'session': '25ff9cbd-12d5-42bb-ad42-85d0a8709dac',
 'username': '',
 'subshell_id': '08f4aed3-23e6-467a-89a0-caa92df520e6',
 'version': '5.2'}

(I am not sure what to make of this, but perhaps someone else would know. I can't even guess whether this might be an upstream bug of some kind.)

[krassowski]

I believe this is related to changes in ipython/ipykernel#1451

Yes, ipykernel (7.0.2) removed reconciliation of output coming from a thread. It now requires manually setting the desired output thread.

Previously this worked:

from threading import Thread
from time import sleep

def child_target():
    for i in range(10):
        print(i, end='', flush=True)
        sleep(1)

def parent_target():
    sleep(1)
    thread = Thread(target=child_target)
    thread.start()
    thread.join()

Thread(target=parent_target).start()

but now this is required:

 from threading import Thread
 from time import sleep
 
+ parent = get_ipython().get_parent()
 
 def child_target():
+    get_ipython().set_parent(parent)
     for i in range(10):
         print(i, end='', flush=True)
         sleep(1)
 
 def parent_target():
     sleep(1)
     thread = Thread(target=child_target)
     thread.start()
     thread.join()
 
 Thread(target=parent_target).start()

Maybe we could make this happen automatically for ipywidgets.Output context manager?

[rawlins]

Thanks, that sounds consistent with my proposed solution. I can work on a PR based on my proof of concept (though it'll be after a conference I'm currently scrambling for).

However, do you have any guesses as to why the current code works correctly on the first output widget and not thereafter? That behavior is what was making me unsure whether I was on the right track...

[krassowski]

@rawlins can you see if #4021 helps?

[krassowski]

@rawlins did you have any chance to test #4021?